1. Field of the Invention
This invention relates to a fixing apparatus used in electrophotography.
2. Description of the Prior Art
In electrophotography, an electrostatic latent image is formed on a photosensitive body having a photoconductive substance, the latent image is developed by a charged toner to visualize the image, the visualized image is transferred on a supporting member, and finally the toner is fixed on the supporting member by heating to obtain a copy. As a fixing system, in the case of using a dry powder developing agent, the supporting member bearing a visualized image is passed through a pair of mutually press-contacted rollers which constitutes a roller fixing apparatus. In general, the pair of rollers consist of a heating roller and a pressure roller, and each of the surfaces of these rollers is provided with a coated film of an offset preventing material such as for example silicone rubber or ethylene fluoride resin (Trade name Teflon) for preventing the offset of the toner image on the supporting member.
However, for improving the fixation, if the surface temperature of the roller is raised or if the offset preventing material on the roller surface is deteriorated, the toner image tends to offset on the heating roller and sometimes, the supporting member will be wound round the heating roller. For preventing this, it is usual to apply an offset preventing liquid such as silicone oil on the heating roller with the aid of, for example, felt or a roller. In the application of the offset preventing liquid, it is necessary to control the amount of the offset preventing liquid on the surface of the heating roller or the pressure roller to a suitable amount.
As a liquid applying material, it is usual to use a material such as heat resisting felt or fabric material having the property of absorbing and maintaining the offset preventing liquid utilizing the capillary action. However, in a system where the liquid applying material is always press contacted with the heating roller, the pressure roller or the liquid applying roller, it is quite difficult to suitably control the amount of liquid to be applied, and in many cases, an excess amount of the offset preventing liquid is applied. Especially, when the apparatus is not used for a long time, the liquid applying member has already absorbed the offset preventing liquid by the capillary action to the saturated state so that an excess amount of liquid is applied. As a result, the toner supporting member (a transfer paper) is contaminated by oil. Further, when the heating roller or pressure roller is formed by silicone series elastic material, if an excess amount of silicone series offset preventing liquid is applied, said roller will be swelled so that the diameter thereof becomes non-uniform. In a system where one of a pair of fixing rollers will rotate following the other roller by friction, the follower roller will slip and the rotating speed of the follower roller varies due to an excess amount of the liquid.